1. Field of the Invention
The present invention relates to antennas and methods for manufacturing them, which reduce overall size, make return loss characteristics broadband and simplify manufacturing process.
2. Description of Related Art
Some types of antennas which are, e.g., attached to a vehicle window, mounted in a vehicle cabin or mounted to a wireless communication device may be required to be invisible to users because of regulations or appearances. Generally, antennas are made invisible to users by using smaller diameter conductor lines or narrower width conductor films for antenna elements. For example, JP-A-2006-140789 (US2006/0109187) proposes an antenna in which a smaller diameter conductor (hereinafter referred to as “linear conductor”) is sandwiched between two visible light transparent insulating films, thereby improving mountability. It also discloses that, in order to solve a problem of increased electrical resistance with decreasing conductor diameter, multiple linear conductors are parallel disposed for lowering electrical resistance, thereby reducing the antenna loss.
FIG. 9 illustrates a conventional antenna 91 in which a plurality of linear conductors are parallel disposed in order to broadband return loss characteristics. In the antenna 91, a plurality of parallel disposed linear conductors 92 are sandwiched between two insulating films 93, and the same side ends of the linear conductors 92 are connected to a feed point 95, thereby causing all of the linear conductors 92 to function as a driven element 96. Further, another plurality of parallel disposed linear conductors 92 are sandwiched between two insulating films 93 and located separately from the driven elements 96, thereby providing parasitic elements 97 of a resonant frequency different from that of the driven element 96.
This antenna 91 has some problems in that the overall size of the antenna tends to increase, and an extra step of forming the parasitic elements 97 is added to the manufacturing process. In addition, if the driven elements 96 and parasitic elements 97 are some distance apart, then the antenna 91 will have directionality, which presents a disadvantage when wishing to broadband an omni-directional antenna.